Up to the present moment, the Bayer process is the only commerical method used to manufacture alumina for the Hall electrolytic cells. The Bayer-Hall process is over 100 years old, and requires feed of good quality bauxite which is of limited global reserve and the price of which has escalated rapidly. Also there is a heavy capital demand due largely to the slow precipitation rate of the hydrate and long 45-72 hour slurry retention time. The combination of high bauxite and capital costs results in high alumina cost. Accordingly, the security and perpetuation of the aluminum industry, especially domestic, and the maintenance of the competitive position of aluminum metal would demand the development of a process that could economically use other alumininous ores, especially domestic.
Considerable research work has been done on processes to make alumina from kaolinitic clay of which there is a virtually unlimited reserve in the USA and worldwide. These processes use various techniques for the extraction of the alumina from the ores, principally by the use of aqueous acid extractions of the alumina such as AlcanPechiney H+ process using hydrochloric and sulfuric acids, which is now being piloted, as well as other processes such as those using nitric or hydrochloric acids on which the U.S. Bureau of Mines is working on pilot plant designs. These processes produce very fine alumina by the hydrolysis and dehydration of hydrated aluminum salts. However, the most promising process is the Toth process which extracts alumina in the form of anhydrous aluminum chloride from clays and other aluminous ores by carbo-chlorination, purifies the aluminum chloride and then oxidizes it to make a finely divided aluminum oxide. However, as with the aforelisted acid processes, there is one obstacle to utilization of the aluminum oxide produced; the fine powders are too light and bulky for economic transport, handling and introduction into the hot Hall cells wherefrom they would be carried away by the offgases.
In addition, the alumina must have the capability of absorbing hydrogen fluoride gases from the Hall cell offgases which is an economically necessary property of alumina today.
Hall cell offgases are passed through a fluid bed containing the feed alumina wherein the hydrogen fluoride is absorbed in the alumina not only removing the HF, a severely corrosive and toxic pollutant, but in addition recovering the very valuable fluorine component in usable form in the alumina feed. Previously large outlays were made for additional cryolite and aluminum fluoride since other pollution control methods did not remove the HF pollutant in usable form. This method for removal of the HF is described in U.S. Pat. No. 3,760,565.
It is a principal objective of the instant invention to produce an alumina that can be made in the aforementioned manner to absorb HF from Hall-cell offgases in a fluid bed.
Up to the time of the instant invention, only Bayer alumina was available to operate satisfactorily in such a mode, so a crying need was obvious for a process to inexpensively convert the finely divided alumina made by other processes than Bayer into alumina having about the same bulk density, particle size and handling characteristics as Bayer alumina and that would also dissolve as readily in cryolite as Bayer alumina and that would also absorb HF in the prescribed fluid beds as well as Bayer alumina without excessive degradation: all this without interfering with the high purity demanded in a Hall cell alumina feed nor with the maximum 1.5% moisture content specification. All this has been accomplished by the instant invention at low cost.
Extensive prior art search indicated no close reference probably because of the unique fine alumina produced in this process and the need to have many specific properties in the final granular product which required the invention of applicant's unique and novel process of agglomeration. French Pat. No. 1,190,094 seems the nearest reference but it applies to making granules of activated alumina from conventional Bayer alumina hydrate particles. The inventor does not make or use alumina hydrate: does not make activated alumina from alumina hydrate, nor does he follow the reference's procedure.